Zero-volt switching (ZVS) is a switching scheme wherein a field-effect transistor (FET) delivering power to a load is switched on or off only when the drain-source voltage is at or near zero volts. Zero-volt switching uses pulse width modulation (PWM), but with an additional separate phase to the PWM timing to allow for ZVS operation. Zero-volt switching enables the voltage regulator to engage in “soft switching,” thus avoiding the switching losses that are typically incurred during conventional PWM operation and timing.
Zero-volt switching can be employed in a variety of power conversion systems that require high switching frequencies at higher input voltages and voltage drops. Wireless power transfer and charging systems are an example of a technology that can benefit greatly from zero-volt switching. The Alliance For Wireless Power (A4WP) is an industry standard group that uses the principles of magnetic resonance to develop a wireless energy transfer system over distance. A4WP wireless power transfer uses directed and controlled magnetic fields to replace traditional power cords. To do this, the transmitter utilizes one or more primary windings in order to induce an even magnetic field above its surface. A receiver in the magnetic field uses a secondary winding which captures the magnetic energy and converts it back to electrical energy. In A4WP wireless power transfer, the switching frequencies are multiples of 6.78 MHz. Accurate zero-voltage crossing (ZVC) detection is the starting point for high efficiency ZVS control. With such high-frequency switching, the timing accuracy of the zero-volt switching needs to be on the order of a nanosecond. Conventional zero-voltage crossing detection methods employ very fast comparators and gate drivers that have very little delay. However, typical comparator delay is already above the aforementioned nanosecond range.